Cellular wireless communications systems such as those defined by the IEEE 802.16 standard (IEEE std. 802.16-2004, approved Jun. 24, 2004) provide communications for mobile systems (MS) (e.g., phones, computers, or other portable devices) over a service provider's core network or backbone by means of base stations (BS) connected to the core network that relay communications to and from the MS via a wireless link. The geographic area over which a particular BS is able to communicate wirelessly (i.e., the air interface) is referred to as a cell. An MS enters the network by attaching to a specific BS with which it is in range, and the attachment may move to other BS's when conditions warrant (e.g., a change of location as the MS moves from one cell to another) with a handover process.
A BS may provide uplink channels and downlink channels for multiple MS's that are attached to it by both time division and frequency division multiplexing. The BS periodically broadcasts a superframe header (SFH) that divides time into discrete segments called superframes that contain time slots used for data transmission between the BS and an MS. A superframe is divided into various other frames and sub-frames down to the basic symbols that are encoded according to a particular data modulation scheme such as OFDM. The SFH and other frame preambles contain control information that allocates bandwidth resources among multiple MS's by assigning particular time slots and frequencies to the MS's for both the uplink and downlink channels. Control information in the form of pilot or training signals is also provided to enable the timing synchronization necessary for symbol detection and, in the case OFDM or OFDMA, for maintaining orthogonality between the sub-carriers.
In order for an MS to attach to a BS, it scans over a number of different frequency channels until it receives an SFH. The MS then synchronizes with the received signal and contends for medium access within a time slot defined for that purpose following an SFH in order to register with the BS. After registration, the MS may enter a power saving sleep mode and operate with a pattern of available intervals and unavailable intervals. The BS provides resource allocation, i.e., defined time slots and frequencies for the MS's uplink and downlink channels. during available intervals. If the MS has no traffic, the MS may de-register from the BS and further enter an idle mode, with a periodic pattern of paging available intervals and paging unavailable intervals. If there is incoming new traffic for the MS in idle mode, the BS pages the idle mode MS during its paging available intervals.
One type of BS, usually operated by the cellular service provider for servicing a relatively large area and many MS's, is referred to as a macro BS, and the area served by the macro BS is referred to as a macro cell. Another class of BS is a femtocell BS having a coverage area or femtocell that is usually very much smaller than a macro cell. A femtocell BS is a low-power BS that is typically installed by a subscriber in the home or elsewhere in order to lower costs and extend service coverage. A femtocell BS usually employs a broadband connection such as fiber optic, cable, or DSL for a backhaul link. A femtocell BS may be public or restricted to only selected subscribers, referred to as a closed subscriber group (CSG).
A femtocell BS may use the same or different frequencies as a macro BS. When a femtocell is at least partially overlapped by a macro cell, the latter is referred to as an overlay macro cell. A femtocell BS listens to overlay macro BS transmissions and employs timing synchronization as well as TDM and FDM for its transmissions to reduce interference with its neighboring macro BS's. For practical reasons, however, some amount of interference with overlay macro BS's may still result. In order to further reduce such interference, as well as conserve power, a femtocell BS may be configured to operate in either a normal operating mode or a low-duty mode (LDM). In the normal operating mode, the femtocell BS operates as described above by periodically broadcasting control information as well as managing scheduled downlink and uplink traffic to and from the MS's that are attached to it. In the low-duty mode, the femtocell BS alternates unavailable and available intervals according to a pre-defined pattern. During an available interval, the femtocell BS is active on the air interface via its wireless subsystem as described for the normal operating mode so it can continue to page the MS's that are attached to it to determine if they are still present in its cell. During an unavailable interval, the femtocell BS ceases all transmitting activity but may continue to listen for transmissions for purposes of synchronization and interference detection. Because no control information is transmitted by the femtocell BS during an unavailable interval, an MS is unable to communicate with it until an available interval occurs.
A femtocell BS may be configured to enter the low-duty mode only if no MS's are currently attached to it or if all attached MS's are either in sleep mode or idle mode. Described herein are methods and systems for reducing the service impact of the low-duty mode on MS's either already attached or are attempting to attach to a femtocell BS.